Only compile steps that are actually being executed

When running partial workflows - locally or particularly in Lightning - compilation can take some time.

Right now, we pre-complile all steps in the workflow before the run starts. This is cleaner architecturally because we ensure the runtime has a fully executable workflow. It also ensures that syntax errors are caught immediately.

The problems come in when:

• You're running a single step from a workflow, but that workflow contains another step with huge mappings
• An orphaned, unused step has a syntax errror

We could add a hook to the runtime which allows it to compile steps just-in-time. This way, only steps that are executed are compiled.

It should probably be an option. The worker/cli has a "compileJIT" flag, defaulting to true.

The runtime is passed a compile callback, which is called on every step.

Either the runtime OR the compile callback itself should cache compilation (maybe find a cheap hash for the code, rather than using the expression as a key, because that's a huge key for large steps). I suppose you could also mark whether the step is compiled or not.

Start by tracing how the worker/cli passes the compile callback into the runtime and how workflow steps are currently pre-compiled. Define the compileJIT option and decide where compilation caching belongs, then verify that only executed steps compile while syntax errors in unused steps do not block the run.